Compressed-air starting is one known mechanism for starting engines. The compressed-air starting includes feeding compressed air into a cylinder with a starter valve, pressing down a piston with the compressed air thus fed, and increasing an engine speed to an engine speed high enough to start the engine.
Conventionally, the starter valve has been oriented toward a combustion chamber in a cylinder head and is arranged in parallel with or orthogonal to an axial direction of the cylinder. Unfortunately, the configuration in which the starter valve is arranged orthogonal to the axial direction of the cylinder involves an extra space (dead volume) formed in the combustion chamber (for example, a starting air discharge path in Patent Literature 1).
The configuration in which the starter valve is arranged in parallel with the axial direction of the cylinder requires a large thickness to separate the starter valve and a cooling water passage from each other in the cylinder head. Thus, the starter valve requires a large space to be disposed in the cylinder head. Reducing a space for providing the starter valve has been an important technical task for downsizing of the engine involving downsizing of the cylinder head.